In any normal television system, the transmission of the wide band video signals which are to produce the actual picture elements on the screen of the receiver, is interrupted between the scanning periods for line and field synchronisation purposes. Consequently there are periods during which no video signals are being transmitted. It is now possible to use these periods for the transmission of data which is not necessarily concerned with the video transmission itself.
Basically, data representable by standard graphical symbols such as alpha-numeric symbols can be transmitted via a restricted channel provided that the rate of transmission is restricted. It is now possible to use periods as aforesaid especially the line times of the field blanking intervals (i.e. the times of the individual lines occurring between fields which correspond with the times occupied by video signals on active picture lines), for the transmission of pages of data. Typically, using 8-bit digital signals representing alpha-numeric characters at a bit rate of 2.5 M bit per second, 50 pages of data each consisting of 22 strips of 40 characters can be transmitted repeatedly in a total cycle time of 90 seconds using only a single line of the field blanking period per field of the 625 line system as operated in the United Kingdom.
Data transmission as described above is already commercially available in the United Kingdom under the name "Teletext."
It is generally accepted that teletext displays should consist of 40 characters per row and ideally for international compatibility 24 rows per page. The U.K. teletext transmission standard specified a data rate of 6.9375 Mbits per second (which has proven to be at the upper reasonable limit of transmission rate for system I, B/G system) so as just to achieve transmission of a complete row of text on one video line of the field blanking time.
The advantage of conveying one row of text on one video line is to achieve maximum economy in requirements for transmission of addressing information needed to correctly position the text information on the displayed page. Since whole rows of text are transmitted on each line, only a row number need be transmitted with each data line of text. Row zero which acts as the page demarcation signal requires additional page numbering information and also incorporates various display and interpretation codes appropriate to the particular page. In order to facilitate parallel magazine working every row of text also incorporates a 3 bit magazine number, being the most significant digit of the page number.
The above structure incorporating as it does one text row on every data line thus results in a very efficient utilization of the transmission facility. Inherent in this teletext transmission system, as just described, is its fixed format nature. Fixed format does not necessarily mean that a certain position of a code on a data line must correspond to a certain position of that character on the screen. When 40 characters are transmitted on a data line however, as in U.K. teletext, it does. Fixed format, more generally, means that each of the 40 character positions on the display screen have been "occupied" by the transmission of one and only one transmission code or alternatively every transmitted character or control code occupies one position on the display screen or equivalently in the display memory. The consequences of fixed format are economy in receiver storage and easy integration towards an error free display in the event of a moderate reception bit error probability.
A further attribute of Teletext is synchronism. By synchronism is meant the fact that control and address codes are synchronized in time to the horizontal sync pulses of the TV system. This means that the teletext decoder has prior knowledge of the position of the control and row address codes with respect to their occurrence relative to the TV sync pulse. These codes do not have to be "sorted out" from a mixed sequence of text and addressing information. In U.K. teletext the hamming-protected addressing control codes are put as soon as possible after line sync near the beginning of the data line signal. This position is directly after the three-byte preamble achieving bit and byte sync of the data receiver; in this position they can never be confused with text information. With U.K. teletext where one row of text occupies one data line this is the obvious and natural way of achieving synchronism.
It is important to note that synchronism, in general, does not mean that one row of text is conveyed by one data line. This latter attribute may be termed "row integrity." Row integrity is achieved by the U.K. teletext system where exactly the contents of one 40-character row of text are transmitted by one data line.
Row integrity is not necessarily a factor which leads to transmission security or easy integration towards error free reception. Row integrity is desirable for economy of address transmission and simplicity of decoder design. But a system without row integrity can still be fixed format and synchronous yet be only minimally more complex than a system like U.K. teletext which possesses all three attributes; row integrity, fixed format and synchronism.
Where the prime request is for a 40-character row yet the TV system is of insufficient bandwidth to allow enough bits per video line to convey 40 characters per line, then row integrity is not possible. A system without row integrity will be described which represents minimal departure from the U.K. teletext system.